We found this circuit in a magazine 73 from the 90's. It is a circuit indicated for radio-amateur transmitters with the frequencies determined by the sets of coils.
This code switch only activates a relay when the right sequence of buttons is pressed. The circuit...
This current-limited variable power supply was found in 1977 book. The diodes are according to...
This circuit for measuring bridges was found in American documentation from the 60's. Transistors are...